The present invention relates to a method of repairing a structure or a component composing internals of a nuclear reactor pressure vessel in a nuclear power plant in service and, more particularly, to a method of repairing a structure or a component irradiated with neutrons and having a crack defect by covering the defect portion of the structure with a cover plate and welding the structure and the cover plate to realize a high reliable repair for the integrity after the repair.
It is worried to occur a crack defect such as a stress corrosion cracking in a structure or a component under a high temperature and high pressure environment in a reactor pressure vessel as time passes. The stress corrosion cracking occurs in a superposed condition of a deterioration factor such as local change in the composition of a material itself, a stress factor of tensile remaining stress applied to the structure due to welding or the like and a corrosion environment factor under a high temperature and high pressure environment, and the crack grows. When the crack grows through the structure or the component, a serious accident may occur in the nuclear plant by some possibility. Therefore, a repairing technology to prevent a through-crack in the structure having the crack is necessary.
As for such a repairing technology to prevent a through-crack, there is a repairing method shown in FIG. 19a, wherein the growth of crack is prevented by covering a region containing a crack 1 with a plate 3 and by filler-welding the edge portion of the plate 3 and the structure 2 utilizing heat input generated by arc or welding arc 6 while a filler metal 5 is being added to isolate the crack 1 from the corrosion environment.
However, the structure irradiated with high energy particle rays such as .alpha.-rays, .beta.-rays, neutron rays or the like contains He generated by nuclear transformation of the component elements of the structure. In a case wherein a structure of such a material containing He is covered with a plate and the edge of the plate and the structure are filler-welded as described above, heat is inevitably input large enough to melt the filler material, a part of the base material and a part of the plate in a conventional welding technology such as arc welding. Therefore, when the structure 2 irradiated with neutrons and having a crack 1 is repaired by filler-welding the cover plate 3 and the structure 2 as shown in FIG. 19a, the heat affected zone 8 due to the weld in the structure around the filler-welded portion 7 of the repaired portion possibly may become a new heat affected zone to produce a crack 9, as shown in FIG. 19b.
The above problem for austenitic stainless steel is described in, for example, Journal of Material Science, Vol. 26(1991), pp 2063-2070. The mechanism of occurrence of a crack is that when welding work is performed with adding heat on the above material of an alloy containing generated He and having a cumulative total amount of neutron irradiation more than 1.0.times.10.sup.20 n/m.sup.2, the He easily diffuses to the crystal grain boundaries of the material by thermal activation due to heating of the welding heat affected zone near the welded portion, and the He voids collected in the grain boundaries gathers to form bubbles having sizes of .mu.m order to decrease the strength of grain boundaries of the material. Further when a tensile stress due to solidification and shrinkage is added to the material after the welding, cracks occur in the grain boundaries in the heat affected zone of non-melted portion.
As for repairing of a structure with a crack which is the object of the present invention, in the repairing method according to the conventional technology of covering the structure with a plate and filler-welding it to the structure, the plate and the structure are closely contacted so that the side surface of the edge portion of the plate and the surface of the structure are in a face-contact state, and the filler wire, the side surface of the edge portion of a plate and the surface of the structure are melted to be welded while the filler wire is being fed and thermal energy is being input. Therefore, there are some cases where the amount of heat input added to the structure becomes locally high. Under such a welding condition, there is a possibility that the He easily diffuses to the crystal grain boundaries of the material by thermal activation due to heating of the welding heat affected zone, and the heat affected zone in the structure becomes a new crack generating portion.
There are various disclosures concerning relations between welding of steel containing He and cracking. They are as follows:
Metallurgical Transaction A. Vol. 21A (1990/9) pp. 2585-2596 [H. T. Lin etc.], which discloses that when GTA arc welding is applied to He containing stainless steel, intergranular cracking occurs in a welding heat affected zone; decrease in welding heat input tends to decrease in cracking sensibility.
Welding Journal (1991/5) pp. 123-132 [S. H. Goods etc.], which discloses relations between He amount and cracking property when large heat input GMA welding is applied to 304 stainless steel containing He, wherein as the heat input becomes smaller, scale of cracking becomes small, as in FIG. 12.
Metallurgical Transaction A Vol. 23A (1992/5) pp. 1021-1032 [S. H. Goods etc.], which discloses welding cracking of irradiated material is due to formation/growth/joining of He bubbles, and wherein mechanism of cracking caused by He is discussed.
Welding Journal (1992/4) pp. 43-51 [E. A. Franco-Ferreira etc.], which discloses relations between welding heat input and cracking in case various He amounts are taken in GTA/GMA welding.
DP-MS-89-41 (DE90-001312) Herium Induced Weld Cracking In Irradiated 304 Stainless Steel (U) by A. K. Birchenall, which discloses a relation between welding heat input and cracking in case various He amount are taken.